1. Field of the Invention
The present invention relates, in general, to magneto resistive read heads and hard disk drives, and, more particularly, to a method and apparatus for providing dynamic correction of thermal asperity transients experienced in hard disk drive read heads with a modified preamplifier.
2. Relevant Background
The demand for improved data storage techniques and systems continues to rapidly grow. Hard disk drives utilizing magneto resistive (MR) heads to read and write data onto one or more spinning magnetic platters or disks are one of the more important and wide spread devices in the data storage industry. Hard disk drives may be used in many applications, including enterprise computer systems, personal computers, set top boxes, audio, video, or television applications, and many other large and small computer devices. Many applications are still being developed, and the uses for hard disk drives are expected to increase.
Generally, a hard disk drive system includes a rotating magnetic disk on which information is recorded with a write head. A read transducer or read head is movably supported adjacent the magnetic disk for reading the prerecorded information from the disk. The read head typically flies above the surface of the disk, being supported by an “air bearing” that is created by the spinning disk and the transducer does not touch the surface of the disk in normal operation.
Large portions of the read heads in use today are magneto resistive (MR) heads. The term “magneto resistance” refers to the change in resistivity of the materials of the head in the presence of the magnetic field induced in the head by the magnetic domains recorded on the disk. The use of MR heads in hard disk drives has significantly increased the areal density or density of recorded data on the disk surfaces. Unfortunately, the use of MR heads has been accompanied with a number of operating problems that can create sources of error that can degrade the achievable performance, e.g., the achievable bit error rate performance, of a hard disk drive.
With the low flying heights of the read head, it is inevitable that the head will collide with small particles or with rougher portions of the disk surface creating a “thermal asperity” that in turn can cause permanent bit errors or data reading errors caused by larger thermal asperity transients in the read signal generated by the read head. A thermal asperity results when a metal particle, disk defect, or the like nearly or actually collides with the MR read head, momentarily raising the temperature of the sensor or head. The heat conducted into the MR sensor then diffuses relatively slowly. The rapid rise in temperature changes the MR resistance and results in a voltage transient in the output from the read head. When superimposed on the normal read signal, the resultant shape shows a rapid rise in voltage followed by an exponential-like decay.
If the disk surface or an asperity momentarily comes closer to the MR read element without touching it, an increase in the cooling effect may occur in the MR head. The resulting change in resistance of the MR head material can also cause a thermal asperity transient similar to that produced by the head heating effects but in the opposite direction.
Efforts have been made, with varying results, to reduce the effects of thermal asperities. Physically, efforts have been made to reduce the flash temperature that results from a collision between the head and the disk or a defect by reducing the dynamic friction, the slider dimensions, and the interaction height. Lowering the interaction height requires smoother disks, fewer “glide escapes”, lower particle count and less contamination and debris. Other physical measures have been taken, as well, including designing the heads to have a high magnetic sensitivity, a low effective temperature coefficient, and a wide track width. Some proposals even include using a second, dummy sensor away from the air-bearing surface of the main sensor to provide a reference against which the output of the main sensor can be compared. Differentially sensed dual stripe heads were also used to partially cancel the thermal asperity effects. However, the industry trends of lowering the flying heights and increasing the slider-disk velocities have offset any improvements that were achieved from these physical efforts resulting in a continued need for a technique of correcting for thermal asperity transients.
In addition to the physical measures, compensation measures or correction circuitry has been provided in the read channel or the read/write controller of the hard disk drive system. Both “on-the-fly” and “re-try” types of compensation within the read channel have been used in an attempt to lessen the impact of the thermal asperity effects. The on-the-fly methods include analog read channel front-end processes aimed at processing the thermal asperity events such that they become invisible to the rest of the channel. In some read channel embodiments, thermal asperity is detected by comparing read signals from the preamplifier with a threshold value, with detection sometimes being enhanced with energy level determinations and disk mapping. High pass filters are then used in the read channel to cut off low-frequency components of the signal produced by the preamplifier, and such filters are used alone or in combination in the read channel with automatic gain control circuits and error correction code circuits. The re-try methods include recovery steps that are implemented at the system level as part of a data recovery procedure. These compensation methods and circuits have not satisfactorily addressed signal shifts caused by thermal asperity transients and often result in undesirable added costs and complexity in designing and manufacturing read/write channels for hard disk drive systems.
Hence, there remains a need for an improved method and apparatus for more effectively correcting for thermal asperity transients associated with the use of MR read heads in hard disk drive systems. Preferably, such improved methods and systems would be relatively simple to implement to control design and manufacturing costs as well as control increases in use of chip real estate or circuit space. Further, it is desirable that such an improved corrective method and apparatus be dynamic rather than tied to a one time mapping of a disk to adjust to thermal asperity transients that vary both in amplitude and in time.